Environmental Engineering Reference
In-Depth Information
5.1 Structure and Function
5.1.1 Electrons, Holes and Space-Charge Regions
Understanding the relatively complicated way that solar cells work requires immer-
sion into the most extreme depths of high physics. The small applied model shown
in Figure 5.1 explains roughly the principle involved. There are two horizontal
levels. The second level is located a bit higher than the fi rst one. The fi rst level has
a large number of small hollows fi lled to the top with water. The water here cannot
move by itself. Now someone starts to throw small rubber balls at the fi rst level. If
a ball hits a hole, the water splashes upwards and ends up on the second level. Here
there are no hollows to contain the water. The second level is therefore inclined so
that the water runs off and reaches the draining groove on its own. This groove is
connected to the second level through a pipe and as the water fl ows through, it drives
a small waterwheel with a dynamo. When the water reaches the lower level, it fi lls
up the hollows again. The cycle can start all over again with new rubber balls.
Figure 5.1
Model illustrating the processes of a solar cell.
However, we want to use solar cells not to produce a water cycle but to generate
electric current to run electrical appliances. Electric current is created from the fl ow
of negative-charge carriers, called electrons. These are the same as the water in our
simple model. The solar cell needs a material in which two levels can be found: one
level in which the electrons are fi rmly affi xed like the water collecting in the
hollows, and a second level where the electrons are able to move freely.
Semiconductor materials normally have precisely these properties. Tiny particles of
light, called photons in physics, correspond to the rubber balls and are able to raise
the electrons to the second level.
Conductors, Non-conductors and Semiconductors
Conductors such as copper always conduct electric current relatively well but
non-conductors such as plastics conduct almost no electricity at all. In con-
trast, semiconductors - as the name indicates - only conduct electric current sometimes,
for example at high temperatures, when fed with electric voltage or when radiated with
light. These effects are used in the production of electronic switches like transistors,
computer chips, special sensors and even solar cells.
